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Late Quaternary paleoenvironments are of particular interest to understand how the Earth System’s climate will respond to the undramatic changes during this period, compared with the broader glacial-interglacial variations. In this study, a shallow sediment core (2.84 m long) retrieved from the Red Sea coastal zone in northern Ghubbat al Mahasin, south of Al-Lith, Saudi Arabia, is used to reconstruct the mid-Late Holocene paleoenvironments and sea level based on a multiproxy approach. Remote sensing data, sedimentary facies, benthic foraminiferal assemblages, δ18O and δ13C stable isotopes, elemental composition and 14C dating were utilized. The stratigraphy of the core shows three distinctive depositional units. The basal pre 6000 year BP unit consists of unfossiliferous fine to medium sand sharply overlain by black carbonaceous mud and peat, suggesting deposition in a coastal/flood plain under a warm and humid climate. The middle unit (6000-3700 year BP) records the start and end of the marine transgression in this area. It consists of gray argillaceous sand containing bivalve and gastropod shell fragments and a benthic foraminiferal assemblage attesting a lagoonal or quiet shallow marine environment. The upper unit (<3700 year BP) consists of unfossiliferous yellowish-brown argillaceous fine-grained sands deposited on an intertidal flat. Both middle-and upper-units stack in a regressive shallowing upward pattern although they may be separated by a hiatus. The overall regressive facies and the stable isotopic data are consistent with a late Holocene sea-level fall and a change to a more arid climate.

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The southern corniche area of Jeddah receives through Al-Kumra effluent the equivalent of 300,000 m 3 of semi-treated sewage. Before 2001, the sewage was directly dumped from an outlet situated at about 1 m above the sea surface. Since 2001, the same volume of municipal wastewater is disposed from underwater diffuser situated at about 3 km south of the old effluent. In order to study the environmental consequences of the dumping site transfer from a sheltered to an open sea area, the region was revisited and sediment samples were collected from the same stations sampled prior to the transfer of the effluent. Samples were analysed for their grain size distribution and their content of organic carbon, Al, Fe, Mn, Cu, Zn, Cr, Cd and Pb. Results were compared to those obtained in a previous study carried out in 1999. The study showed that despite the very pronounced dilution effect, the impact of the effluent is measurable and extends, in situ, north and south directions along the entire study area. The impact is very pronounced in the coastal area as shown by the excessive development of algal production which extends further southward and by the destruction of the mangrove stand. Sediments are becoming enriched in heavy metals where concentrations of Al, Fe, Mn, Cu, Zn and Cr are almost doubled. The present study is the first of its kind to be carried out after the effluent transfer. The authors therefore believe that the system did not yet reach a steady state and will continue to change and develop for several years.